This invention relates generally to photoelectric converting circuits for converting optical signals into electrical signals. Such circuits are used in optical communication systems.
There is well known a feedback type transimpedance convertor, shown in FIG. 2, (PRIOR ART) as a photoelectric converting circuit.
The cathode of a light receiving element 11 is connected to the positive pole of a power supply 12 to apply a reverse bias voltage to the circuit. The anode of light receiving element 11 is connected to the input of an inverting amplifier 13. Light received by light receiving element 11 is converted into an optical output electrical signal, which is inverted and amplified by inverting amplifier 13. The output of amplifier 13 is fed back to the input of the inverting amplifier 13 through a feedback resistor 14. A parasitic capacitance 15 and an input capacitance 16 exist in the light receiving element 11 and the inverted amplifier 13, respectively as shown by a short dashes line in FIG. 2.
Given a voltage amplification factor A of the inverting amplifier 13, a resistance Rf of the feedback resistor, CpD as the value of the parasitic capacitance 15 and CA as the value of the input capacitance in the photoelectric converting circuit, an upper cut-off frequency fc.sub.1 of the frequency characteristics of the photoelectric converting circuit is approximated by the following equation. EQU fc.sub.1 .apprxeq..vertline.A.vertline./{2.pi.(CpD+CA)Rf} (1)
FIG. 3 (PRIOR ART) shows another conventional photoelectric converting circuit configuration wherein the same negative feedback type transimpedance method as that of FIG. 2 has been adopted. In the case of the circuit of FIG. 3, an amplifier 21 providing a gain of 1 is connected between the anode of the light receiving element 11 and the inverted amplifier 13. The output voltage of the amplifier 21 is further applied to the cathode of the light receiving element 11 through a capacitor 22. Moreover, a resistor 23 is connected between the cathode of the light receiving element 11 and the power supply 12.
Light receiving element 11 in that circuit is provided with positive feedback, by a bootstrap feedback arrangement. Since the a.c. voltage on the anode side of the light receiving element is applied to the amplifier 21 with a gain of unity (1) and the output is applied to the cathode of the light receiving element 11 through the capacitor 22, the a.c. potential applied across the light receiving element 11 becomes equal. Consequently, the potential difference between both ends of the parasitic capacitance is always zero. Thus the parasitic capacitance 15 will not affect the frequency characteristics of the photoelectric converting circuit.
Given CA' as the value of the input capacitance 24 of the amplifier 21 shown by the short dashes line, the upper cut-off frequency fc.sub.2 of the photoelectric converting circuit is approximated by the following equation: EQU fc.sub.2 .apprxeq..vertline.A.vertline./(2.pi.CA'Rf) (2)
The comparison of Eqs. (1) and (2) shows the fact that the value CpD of the parasitic capacitance 15 in Eq. (2) has been cancelled and thus fc.sub.2 &gt;fc.sub.1. Accordingly, the photoelectric converting circuit of FIG. 3 is obviously more suitable than that of FIG. 2 for broad-band applications.
FIG. 4 (PRIOR ART) shows a circuit configuration employing a field effect transistor 31 corresponding to the amplifier 21 shown in FIG. 3 (PRIOR ART). The drain terminal of the field effect transistor 31 is connected to the power supply 12 and one end of the resistor 23; the gate terminal to the anode of the light receiving element 11 and one end of the feedback resistor 14; and the source terminal to one end of a source resistor 32, the input of the inverted amplifier 13 and one end of the capacitor 22.
Use of a source follower field effect transistor 31 as the amplifier 21 is widely known to be effective. The source follower circuit of a field effect transistor has a gain close to 1 and besides the gain will never exceed 1. Moreover, the advantage is that the input capacitance of the source follower circuit can be made smaller than that of an emitter follower circuit using a bipolar transistor and thus a wider frequency band is available.
The upper cut-off frequenc fc.sub.2 in the negative feedback transimpedance type photoelectric converting circuit utilizing the bootstrap of FIG. 3 is expressed by Eq. (2) to define the gain of the amplifier at the initial stage as 1 and the output impedance as zero. But that definition is limited to an ideal case. However, the upper cut-off frequency fc.sub.2 in the actual circuit is expressed by the following equations; EQU fc.sub.2 =Au.multidot..vertline.A.vertline./[2.pi.{(1-Au)CpD+CA'}Rf](3-1) EQU Au=Gu/{1+(Zo/Zi)} (3-2)
where Gu=non-loaded voltage gain of the amplifier 21; Zo=output impedance of the amplifier 21; and Zi=input impedance of the inverted amplifier 31.
In the equation (3-2), given Gu=1, Zo=0, then Au=1, and in the equation (3-1), EQU fc.sub.2 .apprxeq..vertline.A.vertline./(2.pi.CA'Rf)
which becomes equal to fc.sub.2 given by Eq. (2).
However, when the source follower circuit by the field effect transistor 31 as the amplifier 21 is used, no conditions satisfying Gu=1, Zo=0 are established, i.e., given Rs as the value of the source resistance 32 of the source follower circuit and gm as the transconductance of the field effect transistor 31, non-loaded voltage gain Gu and the output impedance Zo of the amplifier 21 are approximated by the following equations: EQU Gu=gmRs/(1+gmRs) (4-1) EQU Zo=Rs/(1+gmRs) (4-2)
Substitution of Eqs. (4-1) and (4-2) into Eq. (3-2) gives ##EQU1##
Gives gm=10 mS, Rs=1 K.OMEGA., input impedance of the inverted amplifier Zi=200.OMEGA. as a general example of numerical value, Au=0.63 is obtained from Eq. 5. In case Eq. (3-1) is employed as a reference, the influence of the value CpD of the parasitic capacitance of the light receiving element 11 on the frequency characteristics will not be compensable satisfactorily. Particularly when the operating frequency is higher than tens of MHz, the input impedance Zi of the inverted amplifier 13 will decrease so that the effect of the cancellation of the parasitic capacitance 15 by means of the source follower circuit of the field effect transistor 31 decreases to a greater extent.